1. Field of the Invention
The present invention relates to television signal processing. More particularly, the present invention relates to improved signal processing apparatus and methods for deriving from a standard-bandwidth television signal a signal which when reproduced provides a high-definition-like video image relatively free of artifacts.
2. Description of Related Art
Television systems are limited by their transmission bandwidth. Increased temporal (time domain) or spatial resolution (horizontal domain and/or vertical domain) normally implies increased signal bandwidth. If the horizontal resolution is doubled then the signal bandwidth is doubled. Although vertical resolution may be increased without increasing signal bandwidth if the same television standard is maintained (i.e., the number of scan lines is not increased), if the number of scan lines in the standard is doubled, the signal bandwidth is doubled. Thus, if the horizontal resolution is doubled and the number of scan lines is doubled, then the signal bandwidth is multiplied by 4, and if the temporal resolution is also doubled, then the transmission bandwidth requirement is 8 times higher. Bandwidth is a precious commodity as the electromagnetic spectrum and signal storage devices are limited by nature and technology.
Nearly all existing television systems reduce bandwidth by exploiting human eye-brain psychovisual effects to provide an image appearing to have a higher resolution than would otherwise be possible with the system's bandwidth. Among the techniques that have traditionally been used to minimize bandwidth requirements while trying to maintain viewer perceived signal quality are:
1) Interlaced Scanning--A two to one interlace is used as a compromise between line flicker, and vertical and temporal resolutions. For example, in the present U.S. standard, the NTSC standard, 525 lines are transmitted every 1/30 second (a frame). In the first 1/60 second, half of the lines are scanned and displayed (the first field), while in the next 60th of a second, the balance of the picture is scanned and displayed (the second field) by scanning lines lying spatially in between the lines of the first field. PA1 2) Reduced horizontal and vertical luminance resolution--limiting the horizontal and vertical resolution of the luminance component of the television signal. PA1 3) Use of quadrature modulation to carry two chrominance information components with reduced horizontal chrominance bandwidth--the resulting spectral components are multiplexed in the upper portion of the luminance spectrum or a color television signal. PA1 1) Interlaced Scanning--on modern display screens, which are larger and brighter, the interlaced scanning lines do not blend perfectly and scanning lines become visible, creating an objectionable horizontal line pattern as well as line flicker. PA1 A line doubler with motion interpolation corrects for this defect. Such line doublers are the subject of U.S. Pat. Nos. 4,967,271; 4,989,090; 4,876,596; and 4,982,280. Each of said patents are hereby incorporated by reference in their entirety. PA1 However, line doubling alone is not satisfactory. If proper line doubling is performed, viewers tend to move closer to the screen, as scanning lines are no longer objectionable. Closer proximity to the screen makes other image artifacts and shortcomings quite visible and no longer tolerable to the viewer. Such artifacts and shortcomings include: low resolution, noise, color blurring, cross-color effects and cross-luminance effects. Thus, it is undesirable to perform line doubling alone without also suppressing or reducing other picture artifacts and shortcomings. PA1 2) Reduced horizontal and vertical luminance resolution--larger and brighter displays, particularly with the use of line doubling such that viewers move closer to the screen, reveals inadequate luminance resolution. PA1 These effects are compensated in the receiver by using conventional linear enhancement, such as aperture correction or edge enhancement which provides partial correction. Although linear enhancement tends to make the picture appear sharper, it introduces objectionable pre-shoots and overshoots around the image edges. Such techniques are not visually acceptable when used in conjunction with line doublers, and large bright screens. PA1 Improved forms of non-linear enhancers, which reduce horizontal and vertical transition rise and fall times substantially without pre-shoot, overshoot and ringing, are the subject of U.S. Pat. Nos. 4,030,121; 4,262,304; and 4,847,681. Each of said patents is hereby incorporated by reference in its entirety. PA1 3) Use of quadrature modulation to carry two chrominance information components with reduced horizontal chrominance bandwidth--quadrature modulation color systems, such as employed with the NTSC system in the U.S. introduce objectionable artifacts when decoded by simple means. These artifacts include: PA1 Such defects are significantly reduced by the use of adaptive comb-filter decoding techniques. Adaptive comb filters are the subject of U.S. Pat. Nos. 4,179,705; 4,240,105; 4,504,853; 4,706,112; 4,864,389; 4,893,176; and 4,916,526. Each of said patents is hereby incorporated by reference in its entirety.
These techniques and others have been effective in satisfying television viewers for the past decades, but are becoming unsatisfactory to the present-day viewer, because of improvements in current television displays.
These techniques have weaknesses, which result in artifacts. Such artifacts become more and more objectionable as television sets become larger and have brighter displays. Improved television sets are causing the viewers to be more demanding with respect to picture quality.
As a result of this evolution, attempts have been made to correct these defects in the television receiver and display, after transmission or storage. However, such attempts have been only partially successful. The artifacts resulting from the above-mentioned bandwidth reduction techniques and known corrections for such artifacts include:
a) color blurring due to limited chrominance bandwidth; PA2 b) cross-luminance artifacts, known as "dot crawl" or "hanging dots" due to imperfect elimination of chrominance components from the luminance path; and PA2 c) cross-color components, known as "rainbow patterns," due to imperfect elimination of luminance components from the chrominance path.
Prior art techniques seeking to improve viewer-perceived picture quality in the television receiver and display, after transmission or storage, have been only partially successful. For example, as discussed above, such techniques typically have corrected only some visible picture artifacts while leaving others uncorrected or have enhanced certain aspects of the displayed picture while generating new artifacts. Because the eye is sensitive to the worst aspects of a reproduced image, the enhancement or sharpening of the reproduced image accompanied by the generation of new artifacts and/or the elimination of only some pre-existing artifacts is likely not to improve the viewer-perceived picture quality and may actually degrade the pleasure of the viewing experience.
Thus, there is an unresolved need for television signal processing apparatus and methods for producing from a received standard-bandwidth television signal a high-definition-like video image relatively free of artifacts.